This invention relates generally to a material comprised of two different compositions so that the material exhibits anisotropic properties, the material is comprised of a stiff, wear resistant material and a flexible material, wherein each material has a different modulus of elasticity. More particularly, the present invention includes a seal comprised of such material, so that the seal exhibits anisotropic properties.
Currently, seals, such as rubber o-rings are used in static and dynamic environments including pistons, hydraulics, pumps, fuel systems and brakes. Seals have a well-known tendency to break down and fail due to erosion and nibbling. Erosion is caused by the constant action and movement under varying pressures and conditions which the seal experiences. Nibbling, the breakdown of small pieces of the seal over time is also aggravated due to the pressure placed on the seal. Both erosion and nibbling are exaggerated in high-pressure radial sealing situations. Due to this tendency to breakdown there is great need for a seal made from materials which can withstand high-pressure situations without experiencing erosion and nibbling which cause seals to fail, and in turn can cause machinery to fail leading to production shutdowns while these seals are replaced. A material having a high modulus of elasticity, which increases the stiffness of the material, could alleviate the problems due to erosion and nibbling. However, use of these hard materials leads to a different set of problems. The stiffness of these high modulus materials which enables them to withstand high-pressure situations reduces the elasticity. This loss of elasticity reduces sealing performance in both high and low-pressure situations. Material elasticity provides a sealing force which ensures the seal remains in contact with the piston or shaft and the seal bore during use. This prevents leakage and the breakdown of the sealing action. Lack of elasticity is a problem associated with seals having a high degree of stiffness. Some elasticity is also required to allow for the installation of these seals. Seals, which exhibit the stiffness required to avoid erosion and nibbling, are extremely difficult to install and in some instances installation has been impossible.
These problems lead to the current situation where manufacturers must make a choice between three alternatives. First, they can use seals which exhibit enough stiffness to be wear resistant but do not exhibit good sealing capability and are extremely difficult to install. Second, they can use seals which have enough elasticity to allow for sealing force requirements and installation but are not wear resistant and tend to erode, experience nibbling, and breakdown frequently. Finally, they can use seals having two or three components which must work together to deliver the necessary properties of wear resistance, sealing capabilities and ease of installation. However these seals are prone to develop problems due to the fact two or more components must work together to form the seal. Considering the drawbacks of each of the above alternatives, a better solution is needed.
What is required is a seal, which exhibits the properties of a high modulus of elasticity in the axial direction, that is in the same axis as the piston or shaft, and a low modulus of elasticity, along with elastomeric properties in a second direction, perpendicular to the first direction. The modulus of elasticity being a measure of the softness or stiffness of the material, i.e.: a coefficient of elasticity which represents the ratio of stress to strain as a material is deformed under dynamic load. The higher the modulus of elasticity, the stiffer and more resistant to wear, nibbling, and erosion the material is. Seals having such anisotropic properties, in that the seals exhibit increased stiffness and wear resistance to withstand high-pressure applications in a first, axial direction, while having enough elasticity in a second direction, perpendicular to the first direction, to exhibit good sealing force and easy installation are desired.
Accordingly, the present invention is directed at overcoming one or more of the problems as set forth above.
In one aspect of the present invention a material, having anisotropic properties, is made by joining a hard segment polymer with a soft segment polymer. The hard segment polymer and the soft segment polymer are joined so that the material has a modulus of elasticity in a first direction and a lower modulus of elasticity, along with elastomeric properties in a second direction, perpendicular to the first direction. The hard segment polymer is selected from polymers of the engineering thermoplastic family that are melt processable. These hard segment polymers exhibit a high modulus of elasticity, such a high modulus of elasticity increases the stiffness of the material. The increased stiffness allows the material to withstand high-pressure situations without experiencing erosion, wear, and nibbling effects. This has the desired effect of increasing the lifespan of the material because the hard segment polymer provides greater wear, erosion, and nibbling resistance due to its high modulus of elasticity.
The soft segment polymer is selected from the family of elastomeric polymers used for sealing. These soft segment polymers exhibit a lower modulus of elasticity than the hard segment polymers. Including these soft segment polymers with their low modulus of elasticity and elastomeric properties in the material, ensures the material will be somewhat elastic. Some elasticity is necessary to ensure the material will stretch and retract and/or compress and expand rapidly to approximately its original size. Elasticity also makes installation of the material in various applications easier.
The hard segment polymer and the soft segment polymer are processed in a way that ensures the stiffness provided by the hard segment polymer and the elasticity provided by the soft segment polymer are both present in the finished material, thus ensuring a material displaying the desired anisotropic properties. Any available means which is well known in the art can be used to join the hard segment polymer with the soft segment polymer as long as the means chosen allows the material to have the desired characteristics of stiffness and wear resistance in a first direction, along with elasticity in a second direction, perpendicular to the first direction. One means of joining the polymers would be to blend the polymers without a chemical bond or to crosslink the polymers to form a crosslinked blend. A crosslinking agent can be added to aid in forming this crosslinked blend.
Due to this combination of hard segment polymer and soft segment polymer, the material tends to exhibit anisotropic properties. Anisotropic properties refer to the fact the material exhibits stiffness and wear resistance in a first direction while exhibiting elasticity in a second direction, perpendicular to the first direction. This is possible because when the hard segment and soft segment polymer chains are processed in a laminar flow region of a mechanism for processing the material, such as, but not limited to an extruder, the polymer chains tend to orient along the direction of flow in an elongated form, which leads to a material exhibiting these anisotropic properties. Polymer chains are not stable in this elongated form and therefore the polymer chains will not stay in the elongated form for long. Thus, it is necessary to cool or freeze the material so the polymers will remain in the elongated form and thereby maintain the desired orientation. The cooling or freezing of the material can be accomplished using any fluid medium known in the art. The only requirement being that the temperature of the fluid medium be less than the temperature of the processed material so cooling will take place.
Seals manufactured from this anisotropic material will exhibit the stiffness and wear resistance required to withstand erosion and nibbling in a first direction which is lying in the same axis as the piston or shaft to be sealed, while also exhibiting elasticity in a second direction, perpendicular to the first direction, which ensures easy installation on the piston or shaft. These anisotropic seals are perfectly suited for high-pressure situations in that the anisotropic seals will not breakdown or erode as quickly as seals which are currently used. Another major advantage of these seals is their elasticity, which allows for the easy installation of the seals and once installed ensures a good sealing force between the seal and the piston or shaft.